Field of the Invention
The present invention relates to a light source apparatus having, in particular, a light source that emits pulsed light of which center wavelength is variable, and an information acquisition apparatus using the light source apparatus.
Description of the Related Art
A variety of information on constituent materials of an object can be obtained by irradiating the object with pulsed light and detecting light reflected and scattered by the object, light transmitted through the object, or fluorescence emitted from the object. In recent years, identification of a substance of an object has been researched using the following method: an object is irradiated with two pulsed light beams having a frequency difference corresponding to a molecular vibration frequency, and light generated in the object due to Stimulated Raman Scattering (SRS) and Coherent Anti-Stokes Raman Scattering (CARS) is detected.
As a laser beam source that generates two pulsed light beams having center wavelengths different from each other, a fiber optical parametric oscillator (FOPO) is known. The fiber optical parametric oscillator uses four wave mixing (a kind of optical parametric effect) that occurs in a nonlinear optical fiber. In FOPO, upon reception of power of exciting pulsed light incident on an optical fiber, pulsed light with a wavelength different from that of the exciting pulsed light is generated, and the generated pulsed light is oscillated in the oscillator.
More specifically, when an exciting pulsed light is incident on a nonlinear optical fiber arranged in the FOPO, pulsed light (signal pulsed light) with a wavelength shorter than that of the exciting pulsed light and pulsed light (idler pulsed light) with a wavelength longer than that of the exciting pulsed light are simultaneously generated. A portion of the signal pulsed light or idler pulsed light is fed back and then is guided again to the oscillator synchronously with pump pulsed light to be oscillated. The signal pulsed light, the idler pulsed light, or both of them is/are taken out as an output.
In “Optics Express Vol. 22, No. 18, pp. 21921-21928, 8 Sep. 2014”, there is disclosed a method in which a center wavelength of exciting pulsed light is fixed, and a portion for adjusting the length of an optical path is disposed in an optical oscillator to change an oscillation frequency. The center wavelength of signal pulsed light is changed in a FOPO.
In the above “Optics Express”, the center wavelength of exciting pulsed light is fixed, and therefore the center wavelength of optical parametric gain is also fixed. Accordingly, a wavelength variable range of oscillated signal pulsed light is limited within range of optical parametric gain. Therefore, the range is narrow, and light intensity varies in the wavelength variable range and is not constant. Since the center wavelength of optical parametric gain is changed by changing the center wavelength of exciting pulsed light, the wavelength variable range of the signal pulsed light can be broadened. However, in a case where the center wavelength of optical parametric gain does not coincide with the center wavelength of oscillated signal pulsed light, the following phenomenon may occur: when the center wavelength of signal pulsed light is changed, the light intensity of the signal pulsed light is likely to vary owing to the above-described change in the center wavelength.